Low-cost plastic spherical mirror

ABSTRACT

A method for fabricating a plastic spherical mirror is disclosed. First, a plastic part with a mirror surface supported by a plurality of wall structures is designed. Next, a metal mold including mold for forming the plastic part according the above design specification is provided. A mirror surface of the metal mold is polished to an A1 optical grade finish. The metal mold is heated and chilled to form a curvature of the mirror surface of the metal mold. A plastic material formulation is selected and heated until a melt thereof is obtained. The melt is injected into the mold of the metal mold and the injected melt in the mold is cooled to form the plastic part. A thin layer of a reflective metal coating is deposited on the mirror surface of the plastic part and a protective overcoat is formed on the reflective metal coating.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of the U.S. provisionalapplication Ser. No. 60/839,740, filed on Aug. 23, 2006. All disclosureof this US provisional application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a spherical mirror. Moreparticularly, the present invention relates to a low-cost plasticspherical mirror.

2. Description of Related Art

Glass has been the conventional material of choice for use as aspherical mirror. One of the most important reasons is because plasticstechnologies were not developed as they are today. In other words, thetools and materials were not available as they are today. The metal moldtolerances and the resulting parts can be specified and held in tens ofthousandth of an inch. Materials used today are more sophisticated; theplastics are able to emulate the thermal stability and durabilitysimilar to that of glass, and to endure the type of operating conditionsin the past that only glass could have tolerated.

Glass spherical mirrors are expensive because of the secondaryoperations needed to prepare the mirror surface after it is heat formedor slumped to shape. These secondary operations include annealing,grinding and polishing. The annealing process is used to strengthen theglass so that it is strong enough to undergo the grinding and polishingoperation, as well as adding the additional strength needed to resistbreakage during usage. The grinding and polishing stages are necessarybecause of the limits of the tolerance capabilities of glass formingmolds and the physical nature of glass.

Unfortunately, the grinding and polishing stages require a considerableamount of manual processing for producing a finished product; therefore,they are often considered semi-automated processes.

In addition, glass spherical mirrors also have the serious drawbacks ofbreakage, heavy weight, difficulty in mounting the mirror and expensiveshipping costs. To overcome the limitations and drawbacks of glass, alow-cost version of the glass spherical mirror was developed but it didnot provide an acceptable surface finish, was still very heavy and theresulting cost reductions were not comparable to plastic sphericalmirror. Clearly, what is needed is a method and system for manufacturinga plastic spherical mirror to reduce the weight of a spherical mirror toapproximately one-third that of glass, and for making a low-cost plasticspherical mirror of comparable performance to a glass spherical mirror.

SUMMARY OF THE INVENTION

The present invention provides a method and system for manufacturing alow-cost plastic spherical mirror of comparable performance as that of ahigh quality glass spherical mirror.

In an embodiment of the present invention, plastic injection molding isused for manufacturing a plastic part for a low-cost plastic sphericalmirror. The plastic injection molding method is able to yield highertolerance, improved process control, and high reproducibility.

The metal mold for plastic injection molding is able to hold a tighttolerance for a general envelope dimension of a mirror. The sphericalradius tolerance is also to be held at a tight tolerance. Theaforementioned tolerances are comparable to that of the glass sphericalmirrors.

A plurality of plastic material formulations have been developed inwhich a plurality of performance criteria relating to material strength,thermal stability, water absorption, mold shrinkage, material flow intothe mold, UL recognition, manufacturing considerations, surface density,lubricant content, and scratch resistance may be satisfied. Theselection of the plastic material formulation may be based on the metalmold and part testing results.

Vacuum metallization or vacuum deposition may be used for depositing areflective mirror coating for use as the mirror surface of the sphericalmirror. The metal deposited on the plastic surface preferably has athickness of several microns. The metallization phase is performed andthen a protective overcoat is spray coated onto the metalized surface.The vacuum metalized part has improved quality because of the improvedquality control of the surface of the plastic material that is beingcoated by means of the ability to minimize the amounts of flaws on thesurface of the plastic part resulting from the molding process.

A method for fabricating a plastic spherical mirror according to anembodiment of the present invention includes the following steps.

(a) The plastic part is designed so that a mirror surface is supportedfor the prevention of distortion or twisting, and for satisfying aplurality of optical performance requirements.

(b) The plastic injection gates are precisely placed for ensuring theelimination of remnants or knit lines created by plastic resin flow.

(c) A preferred physical size of the plastic part for satisfying aplurality of optical performance requirements and physical designrequirements is selected.

(d) A plurality of support walls are formed and strategically placed inthe plastic part.

(e) A plastic material formulation is selected so that it isspecifically designed to resist deformation.

(f) the tool may also be made from a preferred grade steel having apreferred polish finish;

(g) The metal mold is heated and or chilled to form an optimal curvatureon a mirror surface.

(h) A thin layer of a reflective metal coating is deposited onto themirror surface of the plastic part.

(i) A protective overcoat is formed onto a metalized mirror surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings.

FIG. 1 illustrates an embodiment of a method for fabricating a plasticpart for the plastic spherical mirror in accordance with the presentinvention.

FIG. 2 illustrates an embodiment of a trimmed plastic part for a plasticspherical mirror in accordance with the present invention.

FIG. 3 illustrates another embodiment of a method for fabricating aplastic spherical mirror in accordance with the present invention.

FIG. 4 illustrates a plastic part fabricated by an injection moldingprocess in accordance with the present invention.

DETAILED DESCRIPTION

The present invention will now be described with reference to theaccompanying drawings, in which exemplary embodiments of the inventionare shown. The invention may, however, be embodied in many differentforms and should not be construed as being limited to the embodimentsset forth herein; rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey theconcept of the invention to those skilled in the art.

In the drawings, whenever the same element reappears in subsequentdrawings, it is denoted by the same reference numeral.

For the sake of convenience of understanding, some key terms and phrasesare first presented.

A “plastic material formulation” may comprise a homopolymer, athermoplastic, a copolymer, a polymer blend, a thermoset, a polymerblend, any one of the above material containing performance additives,fillers, or fibers, or any other similar types of polymer materialformulations.

The “deposition of a reflective metal coating over the mirror surface ofthe plastic part” may be accomplished by vacuum deposition, spincoating, spraying, vacuum metallization, sputtering, or any othersimilar process capable of depositing the reflective metal coating onthe order of several microns.

“Low-cost” may be defined as a favorable cost differential as comparedto glass of the same dimensional configuration for use as sphericalmirrors.

A “glass counterpart” is defined to be a glass spherical mirror of thesame dimensional configuration and possesses equivalent functionalitiesas that of the plastic spherical mirror.

As used herein, the words “may” and “may be” are to be interpreted in anopen-ended, non-restrictive manner. At minimum, “may” and “may be” areto be interpreted as definitively including structure or acts recited.

In an embodiment of the present invention, a plastic injection moldingprocess is used for fabricating the plastic part 20 of a plasticspherical mirror. In the present embodiment of the present invention,the plastic injection molding process is capable of providing adimensional tolerance of +/−0.0001 inch for a parabolic plastic mirrorthat ranges from a diameter of from about five inches to about 36inches, in certain preferred embodiments of the present invention.Although the tooling for the plastic injection molding process isrelatively expensive, the cost per each plastic part 20 manufactured ishowever very low. A plurality of complex geometries is reproducibleusing the plastic injection molding process according to an embodimentof the present invention and may be limited only by themanufacturability of a metal mold.

In an embodiment of the present invention, a metal mold's final finishmay be implemented by means of machining and polishing or other similarfinishing methods capable of producing an adequate finish quality, suchas an A1 grade or a grade that is considered as the finest finishavailable for a plastic part 20.

In the embodiment of the present invention, using the plastic injectionmolding process, plastic parts up to about 36 inches in length may beproduced. The procedures used in the plastic injection molding processare well known in the art; and therefore, detail description thereof isshall not be discussed herein.

In the present embodiment, a metal mold for plastic injection moldingprocess is able to provide plus or minus 0.030 inch tolerance for ageneral envelope dimension and a spherical radius tolerance of plus orminus 0.05% for the plastic part. The aforementioned tolerances arecomparable to the glass spherical mirrors. The metal mold is able tohold a tolerance of about +/−0.0001 inch.

In an embodiment of the present invention, a plurality of plasticmaterial formulations 50 may each be used for fabricating the plasticspherical mirror 10 in which a plurality of performance criteria aresatisfied, such as material strength, thermal stability, waterabsorption, mold shrinkage, material flow into the mold, UL recognition,manufacturing considerations, surface density, lubricant content, andscratch resistance.

In an embodiment of the present invention, the plastic materialformulations 50 may comprise one of the following: optical-gradepolycarbonate, natural-grade polycarbonate, UV-grade polycarbonate,polyetherimide, glass-filled grade polyetherimide, PMMA (acrylic), andother comparable plastic materials having similar performance criteria.The selection of the plastic material formulation 50 may be based on thedegree of precision for the mold tooling as well as experimental resultsfrom part testing.

In an embodiment of the present invention, a vacuum metallization or avacuum deposition process may be used for coating a metal layer over themirror surface 40 of the trimmed plastic part 25 as illustrated in FIG.2 or the plastic spherical mirror treated with an evaporated metalvapor. The thickness of the metal deposited on the plastic surface ispreferably about four to eight microns. The metalizing phase is followedby a spray coating of a protective overcoat on a metalized mirrorsurface 45. The metalized plastic part 30 that has been vacuum metalizedmay possess improved quality because of improved quality control of thesurface of the plastic material that is being coated by means ofminimizing of the amount of flaws that are on the plastic surfaceresulting from the molding process. Furthermore, the metallization is tohave an excellent adhesion with respect to the mirror surface 40 of theunderlying plastic part 20.

Referring to FIG. 1, a method for fabricating the plastic sphericalmirror, in which the plastic part 20 of relatively thin thickness holdsits form/shape after being heated and cooled, may include a plurality ofthe following steps:

Part Design

(a) designing the plastic part 20 so that the mirror surface 40 issupported for preventing distortion or twisting by designing a pluralityof wall structures onto the entire edge of a mirror edge (S100);

(b) designing and placing a plurality of injection gates precisely withthe intent of ensuring the elimination of remnants or knit lines createdby plastic resin flow (S102);

(c) determining a preferred physical size of the plastic part 20 forsatisfying a plurality of optical performance requirements and physicaldesign requirements (S104);

(d) designing and placing a plurality of support walls in the plasticpart 20 so that the final design dimensions of the plastic part 20matches that of a glass counterpart (S106);

Material Selection

(e) selecting a plastic material formulation based upon an ability toresist deformation according to a plastic part 20 quality specification(S108);

Mold Tooling Processing

(f) polishing a mirror surface of the metal mold to an excellent opticalgrade finish (S110);

(g) heating and/or chilling the mold cavity to form an optimal curvatureon the mirror surface of the metal mold (S12);

Mirror Formation

(h) depositing a thin layer of a reflective metal coating on the mirrorsurface 40 of the plastic part 20 (S114); and

(i) forming a protective overcoat onto the metalized mirror surface(S116).

Referring to FIG. 2, an embodiment of a trimmed plastic part 25 for usefor a plastic spherical mirror in accordance with the present inventionis illustrated.

Referring to FIG. 3, in another embodiment of the present invention, amethod for producing the plastic spherical mirror 10, in which theplastic part 20 of a relative thickness sufficient to hold itsform/shape after being heated and cooled, may include a plurality of thefollowing steps:

Part Design

(i) designing the plastic part 20 so that the mirror surface of theplastic part 20 is supported for preventing distortion or twisting bydesigning a plurality of ejector pins, for example, 28 ejector pinsaround the edge of the plastic part 20 serving to allow for part removalfrom the metal mold without distorting the surface geometry or damagingthe mirror surface 40 finish (S200);

(ii) designing a plurality of plastic injection gates and placing theplastic injection gates precisely and evenly in the plastic part (S202);

(iii) determining a preferred physical size of the plastic part 20 forsatisfying a plurality of optical performance requirements and physicaldesign requirements (S204);

(iv) designing and placing a plurality of support walls strategically inthe plastic part 20 (S206);

Material Selection

(v) selecting an optical-grade polycarbonate, polyetherimide, or PMMA(acrylic) as the plastic material formulation for use as the plasticspherical mirror (S208);

Mold Tooling Processing

(vi) polishing the mirror surface of the metal mold to an A1 finish,wherein the metal is fabricated using a high grade steel (S210);

(vii) heating and/or chilling a mold cavity to form an optimal curvatureon the mirror surface of the metal mold (S212);

Mirror Formation

(viii) depositing a thin layer of a reflective metal coating onto themirror surface 40 of the plastic part 20 using vacuum metallization orvacuum deposition at preferably a thickness of, four to eight microns(S214);

(ix) spray coating a protective overcoat on the metalized mirror surfaceof the plastic part 20 (S216); and

Finished Part Inspection

(x) maintaining sphericity on the mirror surface 40 of the plasticspherical mirror at a tolerance of +/−0.05% (S218).

Referring to FIG. 4, a plastic part 20 directly after injection moldingprocess in accordance with another embodiment of the present inventionis illustrated. The ejector pins 50 are disposed around the edge of theplastic part 20 to allow part removal from the metal mold withoutdistorting the surface geometry or damaging the mirror surface 40.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1: A method for fabricating a plastic spherical mirror, comprising:designing a plastic part with a mirror surface supported by a pluralityof wall structures; providing a metal mold including mold for formingthe plastic part according to the design specification provided in step(a); polishing a mirror surface of the metal mold to an A1 optical gradefinish; heating and or chilling the metal mold to form a curvature ofthe mirror surface of the metal mold; depositing a thin layer of areflective metal coating on the mirror surface of the plastic part; andforming a protective overcoat on the reflective metal coating. 2: Themethod of claim 1, further comprising: designing and placing a pluralityof injection gates; and determining a physical size of the plastic part.3: The method of claim 1, further comprising: designing and placing aplurality of support walls in the plastic part so that final designdimensions of the plastic part match with that of a glass counterpart.4: The method of claim 1, wherein the plastic material formulation isselected based upon satisfying a plurality of performance criteriarelating to material strength, thermal stability, water absorption, moldshrinkage, material flow into the mold, UL recognition, manufacturingconsiderations, surface density, lubricant content, and scratchresistance. 5: The method of claim 1, wherein the plastic materialformulation is selected from the group consisting of an optical-gradepolycarbonate, a natural-grade polycarbonate, a UV-grade polycarbonate,a polyetherimide, a glass-filled grade polyetherimide, and a PMMA(acrylic).